The present invention relates to a method for the formation of a superhard protective film of a carbonaceous material on the surface of a variety of articles. More particularly, the invention relates to an improvement in a plasma-induced chemical vapor-phase deposition (CVD) method for the formation of a superhard carbonaceous protective film on the surface of various kinds of substrate articles such as personal ornamental goods, by which the protective film can be imparted with excellent properties without affecting the substrate material even when the substrate material has relatively low resistance against the atmosphere and temperature of plasma.
Needless to say, various kinds of personal ornamental articles as jewelry, such as rings, pendants, earrings, necklaces, bracelets, brooches, necktie pins, cuff buttons and the like, are made from noble metals such as silver, gold, platinum and the like sometimes in combination with pearls and gemstones although imitation jewelry can be prepared from less expensive materials such as non-noble metals and alloys thereof, glass and ceramics, plastic resins and so on. While it is a matter of course in these ornamental articles that the beautiful appearance or, in particular, the high luster is desired to be everlasting or at least not subject to a decrease over years as long as possible, an unavoidable problem therein is that, excepting for diamonds and the like having the highest hardness and highest chemical stability, the luster of the surface is gradually decreased due to scratches and discoloration taking place by the atmospheric influences in the course of their fabrication or during prolonged wearing in direct contact with human skin resulting in lessening of their aesthetic value. In addition, some of metals act as an allergen to cause an allergic dermatitis on the human skin of some people prolongedly kept in direct contact therewith. It is also reported that quality degradation of these jewelry articles is unavoidable more or less even when the article is merely kept within a showcase or show window of a jewelry shop because the article there is usually under strong illumination with fluorescent lamps emitting ultraviolet light. Besides, certain glass- or plastic-made optical articles such as eye glasses as well as contact lenses are desired to everlastingly maintain the clarity which is sometimes decreased due to occurrence of scratches during their prolonged use.
Various proposals have been made in the prior art to solve the above mentioned problems by providing a transparent protective coating film of a carbonaceous material having an extremely high hardness approaching that of diamond. While such a carbonaceous coating film can be deposited on the substrate surface by the so-called thermal or plasma-induced chemical vapor-phase deposition (CVD) method in which a gaseous mixture of a hydrocarbon compound and hydrogen is decomposed to isolate carbon which is deposited on the substrate surface forming a film thereon, none of the CVD methods heretofore proposed can be practically applied to jewelry articles because the low clarity of the thus formed coating film may decrease the aesthetic value of the article and, even worse, denaturation is sometimes caused in the substrate surface as a consequence of the atmospheric conditions and temperature in the CVD process which are not mild enough to ensure stability of the material of the articles.
For example, Japanese Patent Kokai 58-91100 teaches a thermal CVD method for the deposition of a superhard carbonaceous protective film in which a gaseous mixture of a hydrocarbon compound and hydrogen is brought into contact with the surface of a substrate kept at a high temperature of 500.degree. C. or higher so as to effect thermal decomposition of the hydrocarbon compound on the substrate surface to isolate elementary carbon. Further, Japanese Patent Kokai 58-110494 teaches a plasma-induced CVD method in which a gaseous mixture of a hydrocarbon compound and hydrogen is passed through the field of electrodeless discharge caused by microwaves to generate plasma which is brought into contact with the surface of a substrate heated at a temperature of 300.degree. C. or higher so as to effect plasma-induced decomposition of the hydrocarbon compound on the substrate surface to isolate elementary carbon which is deposited on the substrate surface forming a film. In this prior art method, the microwaves generated in a microwave oscillator are introduced through a waveguide duct into a reaction chamber in which the hydrocarbon compound at an appropriate low pressure is converted into plasma and decomposed thereby to deposit a carbonaceous film having a hardness approaching that of diamond. The microwaves introduced into the reaction chamber serve also to heat the substrate by the electromagnetic induction.
These prior art CVD methods are limited relative to the material of the substrate article and not applicable to most of the jewelry articles because of the low clarity of the thus deposited carbonaceous films or discoloration of the substrate surface. In the conventional thermal CVD method, the temperature of the substrate surface sometimes exceeds 730.degree. C. so that, when the article is made from a 22-karat gold alloy, a loss is caused in the luster of the article. When an opal is subjected to the conventional plasma CVD method, discoloration or loss of luster is unavoidable on the surface of the opal as a consequence of the excessively high temperature at which the substrate is subjected to the plasma-induced deposition of the carbonaceous film.